Nitric oxide (NO) is a small reactive radical that plays a significant role in cancer biology. Aberrant production of NO and other oxidants is found in various types of tumors, including breast cancer, and also at sites of chronic inflammation, which have been associated with increased risk of human cancers. In addition, NO and other oxidants have been implicated in causing neuronal damage during ischemic stroke. Humans can regulate NO production by using endogenous inhibitors of NO synthase, monomethyl- and dimethylarginine. The concentrations of these inhibitors are controlled, in turn, by two tissue specific isoforms of dimethylargininase, DDAH-1 and DDAH-2, which hydrolyze these inhibitors to remove inhibition of NO production. These two DDAH isoforms represent attractive targets for pharmacological manipulation of NO, but not much is known about how these enzymes work, how they respond to oxidative and nitrosative stress, and whether they can be inhibited selectively by small molecules. This application has three specific aims 1) to determine the catalytic mechanism of DDAH, 2) to determine whether DDAH can be regulated by biologically relevant reactive oxygen or nitrogen species, and 3) to develop inhibitors of DDAH. These studies will be completed on purified proteins with an eye toward determining functional differences between isoforms that would impact their physiological roles and that can be exploited for design of selective inhibitors. By understanding the chemistry behind DDAH catalysis and regulation, we will develop new biochemical tools with therapeutic potential, and will learn more about the role that DDAH plays in cancer biology and in the general response to oxidative and nitrosative stress. Relevance to Public Health: This application studies a key control point for the production of nitric oxide, a reactive chemical that can cause significant health problems such as promoting tumor growth in certain cancers and causing brain damage during stroke if it is not properly regulated. An understanding of the chemistry behind how this control valve works will allow us to understand how humans react to stressful physiological conditions, and will give us new biochemical tools that can be later developed into novel therapeutics.